1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each one embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are used for a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique for forming a transistor using an amorphous oxide containing In, Zn, Ga, Sn, and the like as an oxide semiconductor is disclosed (see Patent Document 1).
In addition, a technique for forming a self-aligned top-gate transistor using an oxide thin film is disclosed (see Patent Document 2). Furthermore, a technique for forming a top-gate semiconductor element including an oxide semiconductor that has a short channel length L and can be miniaturized is disclosed (see Patent Document 3).
The use of a semiconductor film that has a crystalline structure (typically, a polysilicon film, a microcrystalline silicon film, and the like) and is formed over an insulating surface for an active layer of a transistor has been developed actively. Transistors including polysilicon films can form various functional circuits because of its high field-effect mobility.
For example, in an active matrix liquid crystal display device, a pixel circuit for performing image display for each functional block and an integrated circuit including a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like, which are based on a CMOS circuit, are formed over one substrate. In these circuits, a voltage is applied to liquid crystals to drive a pixel portion including a pixel transistor, which functions as a switching element, and a capacitor.
Furthermore, a display device used by being worn on the human body such as a display device worn on the head (a head-mounted display device or a glasses-type display device) or a display device worn on the arm (a watch-type display device or a wristband-type display device) has been proposed. As described above, a variety of display devices have been proposed; thus, users want a display device having a high display quality, that is, a high-resolution display device.
For example, in an active matrix liquid crystal display device, development of expanding an effective screen region in a pixel portion (also referred to as an aperture ratio) has been advanced. In order to make the area of an effective screen region larger, it is necessary to make the area occupied by pixel transistors arranged in a pixel portion as small as possible. In addition, in order to reduce manufacturing costs, developments for forming a driver circuit on the same substrate as that of a pixel portion have also been advanced.
In order to make an area occupied by a transistor as small as possible, some techniques for miniaturizing the line width of a gate electrode of the transistor have been proposed. For example, a method for forming a sidewall-shaped gate electrode that is formed at a step of a gate insulating film of the transistor has been proposed (see Patent Document 4).